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Mechanical disc brakes and cable pull

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Mechanical disc brakes and cable pull

Old 07-24-19, 11:31 AM
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Mechanical disc brakes and cable pull

We have a 2004 Cannondale tandem with Avid BB7 mechanical disc brakes. The levers are Ultegra 6510 STI. Brakes have new pads, and the rotors, though not dead perfect, are quite true. Just a sliver of light showing around pads. Nonetheless, I have to pull the lever nearly to the bar to fully engage the brakes.
I have thought about adding travel agents, but they seem to be discontinued and unavailable.
I am not wild about the Ultegra levers anyway and have thought about changing to bar end shifters, in which case I have some 105 aero levers to use. I am wondering if they would have better cable pull than the STI levers - or are all Shimano levers about the same?
Other ideas? I would prefer to keep the Avids if possible.
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Old 07-24-19, 11:41 AM
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BB7 Road or BB7 Mountain?
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Old 07-24-19, 11:49 AM
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What are the rotor diameters?

Mechanical disks are generally far mushier feeling at the lever then well set up rim brakes are. Add in the tandem's aspects and it's no surprise that you end up nearly bottoming out the levers, at least the rear's.

Why the need to retain the BB7s? There are other mechanical disk calipers with very good reps for stiff feel and overall power. Andy
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Old 07-24-19, 12:04 PM
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Originally Posted by cobba View Post
BB7 Road or BB7 Mountain?
Road
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Old 07-24-19, 12:10 PM
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Compressionless cable housing?
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Old 07-24-19, 12:23 PM
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Originally Posted by Andrew R Stewart View Post
What are the rotor diameters?

Mechanical disks are generally far mushier feeling at the lever then well set up rim brakes are. Add in the tandem's aspects and it's no surprise that you end up nearly bottoming out the levers, at least the rear's.

Why the need to retain the BB7s? There are other mechanical disk calipers with very good reps for stiff feel and overall power. Andy
Rotors are 200mm.

Itís not that I love the BB7ís, but itís a second tandem and I just bought new pads for them. Trying not to over-invest. The bike stops; I guess I am just used to the feel of rim brakes.

Any insight about how the pull would compare between the 105 levers and the current Ultegra ones?
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Old 07-24-19, 12:26 PM
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Originally Posted by cobba View Post
Compressionless cable housing?
Good question. I will have to check, but I strongly expect itís whatever came stock on the bike. The rear run is mostly bare cable.
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Old 07-24-19, 12:45 PM
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Originally Posted by due ruote View Post
though not dead perfect, are quite true
Get them as true as possible and get the pads as close to the rotor without rubbing.

Rotor truing:
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Old 07-24-19, 01:02 PM
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Originally Posted by due ruote View Post
Any insight about how the pull would compare between the 105 levers and the current Ultegra ones?
This may be helpful. I have 5800 levers and BB-7 road calipers on my commuter bike. I've read that from a brake lever cable pull perspective, 5700 and 5800 brakes are mutually compatible.

The brake response isn't as crisp as with 105 rim brakes, that's for certain. But I will say that my disc braking on the commuter is adequate even when the bike is loaded. I try to set the cable tension with as much "pre-load" on the disc caliper arm as I can manage without rotor rub. And I'm fussy, so I'll spend the time truing the rotor so that I can set the pads close.

From your description of "sliver" of light on each side of the rotor, I can't tell how closely you have them set up. Good luck. Phil
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Old 07-24-19, 01:09 PM
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Originally Posted by Andrew R Stewart View Post
Why the need to retain the BB7s? There are other mechanical disk calipers with very good reps for stiff feel and overall power. Andy
Hey Andy, can you elaborate ? I've been considering upgrading the tektro calipers on my humble hybrid project, and a set of BB7's had been the front runner. What else should I be looking at, and are they comparable in price ?
Thanks.......
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Old 07-24-19, 01:17 PM
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I have trued the rotors as shown in the video posted below. The pads are set as close as I can get them without rubbing. Thatís how I narrowed it down to a cable pull issue. But this is my first venture into disc brakes, so I could be missing something. I appreciate the feedback.

(edit) @Phil_gretz, to be clear, the 105 levers I am contemplating using are not STI, but plain aero levers. I believe they are BL-1055.

Last edited by due ruote; 07-24-19 at 01:28 PM.
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Old 07-24-19, 01:22 PM
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The brake levers need to match the cable pull on the calipers, and here comes a possible problem, the cable pull changed between 6600 and 6700 (a generation after your 6510's) and although this was small, and would not be noticeable with rim brakes, the small difference is probably more noticeable (as your finding) with disc brakes (and probably mini v's)

If you looking to go bar-end shifters, you could keep you BB7's and run with drop bar brake levers, say like a Tektro RL520.

If you're looking to do this, you then run into the issue of getting good /cheap 9-speed bar end shifters, you only the Dura Ace option from Shimano, which aren't cheap (but are very good) or Microshift for a lower budget.

Heres a thread you may also be able to pull some useful info from Road disc brake recommendation for old Shimano levers?
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Old 07-24-19, 01:56 PM
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Also make certain that the brake cable housing ends are properly prepped and with the correct metal ferrules installed where required. https://www.sheldonbrown.com/cables.html#cutting

If they are not flat and well-fitting they will compress under stress, lengthening the pull required. You might also consider inline cable adjusters to precisely dial-in the cable tension just short of the pads rubbing.

And to jimc101's comment, Microshift products are good performers (I perceive no difference from Shimano) and good value.

You might also look at Gevenalle's brake/shifter offerings: https://www.gevenalle.com/
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Old 07-24-19, 02:50 PM
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Originally Posted by Andrew R Stewart View Post
Mechanical disks are generally far mushier feeling at the lever then well set up rim brakes are. Add in the tandem's aspects and it's no surprise that you end up nearly bottoming out the levers, at least the rear's.

Why the need to retain the BB7s? There are other mechanical disk calipers with very good reps for stiff feel and overall power. Andy
andrew has much more experience than I do, but my experience with mtb BB7s reflect this comment. My wife has had the same reaction as you, but Ive purposely set up her BB7's so they dont engage too early, as my fully loaded touring experience in very mountainous terrain has shown me that the braking power vs how they feel (compared to our lifetime with rim brakes) is totally adequate, even if our instincts say no.

now of course, we dont know what clearance you have, and to be honest, again from my experience getting used to disc brakes, the clearances are really really small for proper operation, and I have to put on my reading glasses to adjust stuff properly, and put something white on my garage floor to squint and see the clearance properly--ie, a smidge here and there makes all the diff, and Im always surprised by how small a smidge makes the diff.

so you do have to be very observant and adjust how detailed you are in doing stuff.
good luck, and get help from a good mechanic, I did and it helped a lot.
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Old 07-24-19, 03:19 PM
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Originally Posted by cobba View Post
Compressionless cable housing?
+1 If original 2004, you probably do not have compressionless brake housing. It will be an improvement.
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Old 07-24-19, 11:18 PM
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Maybe apples and oranges, but I have 2013 Culprit road bike with TRP Spyre mechanical discs operated by Ultegra Di2 levers. They were original to the bike, and work great. No modulation issues or "spongy" feeling, easy to adjust, and stop very well. The TRPs have an internal spring to keep the pads from dragging on the rotors. Been over 6 years now, and never had a problem with them. I'm running 160 front and 140mm rear rotors, both TRP and Magura (on a spare wheelset). Mine date from 6 years ago, not sure what the current TRP models are, but worth checking out. Mine came with compressionless brake housings, though I've never changed cables or housings.

Last edited by Slightspeed; 07-24-19 at 11:23 PM.
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Old 07-25-19, 03:18 AM
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Originally Posted by due ruote View Post
I have thought about adding travel agents, but they seem to be discontinued and unavailable.
You can get them from here:https://www.rosebikes.se/xtreme-pro-..._size=Standard
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Old 07-27-19, 02:11 PM
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Just in case: do double check the brake model. You need a "Road" one, not the "MTB" ("MTN").

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